Electrically driven brake booster
1. Field of the Invention
The present invention relates to a brake booster, and more particularly, to an electrically driven brake booster.
2. Description of the Prior Art
An electrically driven brake booster is known in the art as disclosed in Japanese Patent Publications No. 9,964/1994 and No. 104,448/1994. The disclosed booster comprises an input member disposed for back-and-forth movement in operational association with a brake pedal, an output member disposed in operative association with the piston of a master cylinder, drive transmitting means for translating a rotating motion of a motor into a linear motion which is transmitted to the output member, and a controller for controlling the motor in accordance with a variable representing an amount by which the brake pedal is maneuvered. The drive transmitting means in this booster comprises a ball-and-screw mechanism which translates the rotating motion of the motor into the linear motion of the output member, thus integrally coupling the input member and the output member.
However, the ball-and-screw mechanism uses an increased number of parts and is complex in arrangement, resulting in disadvantages of an increased weight and a poor transmission efficiency.
In the above disclosures, because the input member and the output member are coupled together integrally, in the event the motor fails for some reason, there results a disadvantage that the master cylinder cannot be directly operated by a driver maneuvering the brake pedal.
A solution to the issue of failure of the motor is proposed in Japanese Laid-Open Patent Application No. 175, 376/1997 where an input member and an output member are arranged so that their axes run parallel and in which the input member has an operational association with the brake pedal and the master cylinder while the output member is arranged to engage the input member only during the forward drive.
Thus, when the motor operates normally, the output member is engaged with the input member, to which a propelling force can be applied. If the motor should fail, though the propelling force cannot be applied, the master cylinder can be directly operated only through the input member in response to the depression of the brake pedal by the driver.
However, the electrically driven brake booster of this cited Application suffers from a further complexity in arrangement and added weight as compared with the boosters disclosed in the above Publications.
There also remains another problem in either disclosure that a result of boosting operation by the booster cannot be correctly fed back to the brake pedal because of the operational association between the input shaft and the piston of the master cylinder.
More specifically, in these electrically driven brake boosters, an input applied by the driver to the brake pedal is directly reflected as a reaction, and the driver has no way of determining whether or not the booster is operating in a correct manner, thus hindering a correct brake operation.
In addition, the disclosed arrangements set up a booster ratio of the electrically driven brake booster in an electrical manner by the controller which controls the motor, but this poses a problem in respect of the reliability as compared with a mechanical set-up of the booster ratio which is effected in a conventional oil pressure booster or the negative pressure booster.
In view of the foregoing, it is an object of the present invention to provide an electrically driven brake booster of a reduced weight and a simple construction and having an improved transmission efficiency while allowing a master cylinder to be directly operated by an operation of a brake pedal if a motor fails.
It is another object of the invention to provide an electrically driven brake booster which enables a correct brake operation on the basis of a brake reaction.
According to a first aspect of the invention, there is provided an electrically driven brake booster including an input member disposed for back-and-forth movement in operational association with a brake pedal, an output member disposed in operative association with the piston of a master cylinder, drive transmitting means for translating a rotating motion of a motor into a linear motion to be transmitted to the output member, and a controller for controlling the motor in accordance with a variable representing an amount by which the brake pedal is maneuvered, the drive transmitting means comprising a rack formed on the output member and a pinion disposed in operational association with the motor and in meshing engagement with the rack.
According to the first aspect of the invention, the drive transmitting means which comprises the rack and the pinion has a reduced weight and a simple construction in comparison to a conventional drive transmitting means which comprises a ball and a nut, and has a better transmission efficiency. If the motor fails, the master cylinder can be directly operated by maneuvering the brake pedal.
According to the second aspect of the invention, the provision of reaction transmitting means which transmits a reaction from the master cylinder to the output member and the input member at a given proportion enables an accurate brake operation on the basis of a brake reaction transmitted to the input member.